feat(events): A.4 — emit SnapshotChanged from the watchdog probe

Adds a new variant to the lifecycle event catalogue so the UI can
auto-refresh the channel/client tree without an independent polling
timer on the Dart side. The supervisor's existing 5 s snapshot probe
is the source of truth: it already pulls a full snapshot to keep
the watchdog honest, so we piggyback on it.

* `chanora_core::SessionEvent::SnapshotChanged { channels, clients }`
  carries the latest channel and client counts.
* The supervisor compares the probe result to `last_counts` and
  fires the event only when the count actually changes. `last_counts`
  is reset to `None` on a successful reconnect so the freshly
  dialled session re-emits its initial counts.
* `chanora_bridge::api::BridgeEvent::SnapshotChanged` is the
  cross-bridge mirror.
* Flutter routes the event through `_onEvent`, which calls
  `_onRefresh()` to repopulate the snapshot view.

The probe-driven detection has known limits — pure within-channel
client moves do not change the count and so are not surfaced. That
gap will close when the supervisor tracks a content hash in
addition to the count; the count-only signal is sufficient for the
common "someone joined / someone left" case observed on cn.teamspeak.app.

crates/chanora_bridge/src/api.rs

@@ -359,6 +359,15 @@ pub enum BridgeEvent {
     AudioStarted,
     /// Audio engine stopped.
     AudioStopped,
+    /// Snapshot probe observed a change in channel/client counts.
+    /// UI uses this to drive an auto-refresh without active
+    /// polling.
+    SnapshotChanged {
+        /// Latest channel count.
+        channels: u32,
+        /// Latest client count.
+        clients: u32,
+    },
 }
 
 impl From<chanora_core::SessionEvent> for BridgeEvent {
@@ -380,6 +389,9 @@ impl From<chanora_core::SessionEvent> for BridgeEvent {
             }
             chanora_core::SessionEvent::AudioStarted => BridgeEvent::AudioStarted,
             chanora_core::SessionEvent::AudioStopped => BridgeEvent::AudioStopped,
+            chanora_core::SessionEvent::SnapshotChanged { channels, clients } => {
+                BridgeEvent::SnapshotChanged { channels, clients }
+            }
         }
     }
 }

crates/chanora_bridge/src/frb_generated.rs

@@ -612,6 +612,14 @@ impl SseDecode for crate::api::BridgeEvent {
             5 => {
                 return crate::api::BridgeEvent::AudioStopped;
             }
+            6 => {
+                let mut var_channels = <u32>::sse_decode(deserializer);
+                let mut var_clients = <u32>::sse_decode(deserializer);
+                return crate::api::BridgeEvent::SnapshotChanged {
+                    channels: var_channels,
+                    clients: var_clients,
+                };
+            }
             _ => {
                 unimplemented!("");
             }
@@ -878,6 +886,12 @@ impl flutter_rust_bridge::IntoDart for crate::api::BridgeEvent {
             }
             crate::api::BridgeEvent::AudioStarted => [4.into_dart()].into_dart(),
             crate::api::BridgeEvent::AudioStopped => [5.into_dart()].into_dart(),
+            crate::api::BridgeEvent::SnapshotChanged { channels, clients } => [
+                6.into_dart(),
+                channels.into_into_dart().into_dart(),
+                clients.into_into_dart().into_dart(),
+            ]
+            .into_dart(),
             _ => {
                 unimplemented!("");
             }
@@ -1055,6 +1069,11 @@ impl SseEncode for crate::api::BridgeEvent {
             crate::api::BridgeEvent::AudioStopped => {
                 <i32>::sse_encode(5, serializer);
             }
+            crate::api::BridgeEvent::SnapshotChanged { channels, clients } => {
+                <i32>::sse_encode(6, serializer);
+                <u32>::sse_encode(channels, serializer);
+                <u32>::sse_encode(clients, serializer);
+            }
             _ => {
                 unimplemented!("");
             }